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Planet “lighter than cork” baffles
Sept. 14, 2006
Courtesy Harvard-Smithsonian Center for Astrophysics
A newfound planet estimated
to be lighter than a giant ball of cork is baffling astronomers.
They’re guessing that some unknown mechanism heats this type of
world internally, puffing it up.
Using a network of small automated
telescopes, researchers at the Harvard-Smithsonian Center
for Astrophysics in Cambridge, Mass., identified the planet.
Designated HAT-P-1, they said it orbits one of a pair of distant
stars in the direction of the constellation Lacerta.
Artist's concept of
HAT-P-1. (Courtesy David A. Aguilar, CfA)
“We could be looking at an entirely new class of planets,”
said the center’s Gaspar Bakos, who designed and built the telescopes,
known as the HAT network. He is also the lead author of a paper
submitted to the Astrophysical Journal describing
About 1.38 times wider than Jupiter, HAT-P-1 is the biggest known
planet, yet weighs only as much as half of Jupiter, the Bakos
added. That would make it one fourth as dense as water.
“In other words, it’s lighter than a giant ball of cork! Just
like Saturn, it would float in a bathtub if you could find a tub big
enough to hold it, but it would float almost three times higher.”
It circles its host star every 4.5 days in an orbit one-twentieth of
the Earth-Sun distance, the astronomers said. Once per orbit,
they added, it passes before its parent star. This dims it by about
1.5 percent to our view. Planets that darken their parent stars in
this way are called transiting planets.
This star is part of a double-star system called ADS 16402, visible
Although perhaps stranger than any known world outside our solar
system, HAT-P-1 is not alone in its low density, researchers
said: the first transiting planet ever found, HD 209458b, also
is puffed up about 20 percent larger than theory predicts, while
the new one is 24 percent larger.
“Out of 11 known transiting planets, now not one but two are substantially
bigger and lower in density than theory predicts,” so the
previous find can no longer be dismissed as a fluke, the center’s
Robert Noyes said.
“This new discovery suggests something could be missing in our
theories of how planets form,” he continued. One way to explain
the find is that some unknown mechanism provides certain planets
with more internal heat than previously believed, he added.
The HAT network consists of six telescopes that conduct robotic observations
each clear night. They seek out transiting planets. Astronomers
measure a planet’s size from the amount of the star’s dimming.
Combined with the mass—determined by measuring the amount of
the star’s wobble as the planet orbits it—researchers then
calculate a planet’s density. The newfound world is estimated
to be 450 light-years away; a light-year is the distance light travels
in a year.
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